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Mathematicians in Scotland calculate that "self-replicating" alien probes could already have explored our solar system and may still be here but undetectable to our current technologies.

Drs Arwen Nicholson and Duncan H. Forgan from the University of Edinburgh had previously calculated that if a Voyager-sized probe passing through the galaxy picked up speed using slingshots around stars it could travel 100 times faster than otherwise.

The slingshot technique uses the gravitational field of stars or planets to "slingshot" a craft and boost its speed. The Voyager 1and 2 probes launched by NASA in 1977 used slingshot maneuvers around the planets they passed to pick up speed, and Nicholson and Forgan calculated that interstellar probes could use the same technique around stars.

The new calculations, reported in the International Journal of Astrobiology this week, expanded on the previous work by using "self-replicating" probes in the computer models to see how the self-replication would affect the timescale.

The robotic probes could explore our galaxy and self-replicate themselves from interstellar dust and gas, after which the parent and child probes would each set off for a different star, where they would look for signs of life and then self-replicate themselves again. The probes would therefore disperse themselves radially across space.